In some applications, membrane exchange humidifiers typically include a water-permeable and/or water vapor-permeable membrane. A gas stream which requires humidification, or dry stream, flows over one surface of the membrane, whereas a moisture-laden gas stream flows over the opposite surface of the membrane. Moisture from the moisture-laden stream passes through the membrane and enters the dry stream.
In some humidifier systems, such as fuel cell applications, for example, the ionic conductivity of a polymer electrolyte membrane, and therefore, the performance of the system, depends on the hydration level of the membrane. Consequently, in a fuel cell system, the fuel gas stream, the oxidant gas stream or both may be humidified to sufficiently hydrate the polymer electrolyte membrane and sustain optimum performance of the fuel cell. In some fuel cell applications, an air stream directed into the fuel cell is humidified by the moisture-laden cathode exhaust stream discharged from the cathode of the fuel cell. However, it is typically necessary to humidify the oxidant gas stream in order to optimize fuel cell performance.
The capability of the reactant gases to absorb water vapor varies with temperature and pressure. If a reactant gas stream is moisturized at a temperature which exceeds that of the operating temperature of the fuel cell, moisture has a tendency to condense on surfaces as the gas stream enters the fuel cell. This may lead to flooding of the electrodes, thereby adversely affecting fuel cell performance. On the other hand, if the reactant gas stream is moisturized at a temperature which is lower than the operating temperature of the fuel cell, the reduced water vapor in the reactant gas stream has a tendency to dehydrate and damage the polymer electrolyte membrane. Therefore, it is preferable to moisturize a reactant gas stream, typically at least an oxidant gas stream, at or close to the operating temperature and pressure of a fuel cell.